1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
Recent years have seen development of color printers, digital color multifunction apparatuses, or the likes which form images using page description language (PDL) files generated and sent by, for example, host computers.
These image processing apparatuses interpret received PDL files so as to generate raster image data and attribute information which represents the attribute of the image. Image attributes are classified in units of pixels into, for example, a character/line drawing area, a figure area, and a natural image area. A number of techniques are introduced which utilize attribute information to effect image processing for obtaining output images using image processing apparatuses. For example, in one of these techniques, pieces of object information which represent image elements are generated. Then, the image processing is performed which is appropriate for the characteristics of the image elements.
On the other hand, another type of image processing apparatus such as a copying machine generates digital image data from a read original image and performs image processing necessary for printing out. In this type of image processing apparatus, image attributes of the digital image data are determined in units of pixels or units of blocks in the data. Then, appropriate image processing is performed in accordance with the result of the determining operation so that a high quality printed output is obtained. For example, a technique is applied to this type of image processing apparatus, in which image data generated from a read original image is analyzed, and at least two areas, e.g. a character area and a grayscale image area, are discriminated from each other. Then, a first and a second image processing operations are selectively performed on the individual areas. These first and second image processing operations include a technique which selectively smoothes or sharpens the character area or the grayscale image area into an intended level. The first and second image processing operations also include color conversion processing such as color collection and under color removal, and image editing processing such as mosaic image generation and mirror image generation. In addition, these image processing operations can be configured such that parameters necessary for the operations are manually set.
Moreover, techniques have been put into practice which apply the above described technique for determining image attributes (image area separation processing) so as to process PDL files, which brings about further increased output quality. For example, a technique is introduced in which image area separation processing is performed on each pixel in a raster image generated by interpreting a PDL file. Then, the result of the image area separation processing and attribute information generated by interpreting the PDL file are referred to in units of pixels, so that processing schemes suitable to each of the pixels are determined. Furthermore, there is another known technique for limiting signals used in an image separation processing operation so as to prevent image deterioration from occurring due to determination errors in the image area separation processing.
The above mentioned attribute information generated from a PDL file provides accurate segmentation of objects such as a character/line drawing object, a figure object, a natural image object, etc. However, such attribute information is sensitive to an application used in generating PDL files, and thus can inconsistently vary depending on the type and version of the application.
On the other hand, image area separation processing permits determination of attribute information which is consistent in an image processing system and also permits image classification most suitable to the system. However, image area separation processing involves a possibility of determination error due to its pixel by pixel processing. This error can cause a pixel processed for a character object to appear in a part of a natural image, for example, which results in image degradation.
A technique which utilizes attribute information generated from both a PDL file and a result of image area separation processing also involves a risk of failure in image reproduction: in a case where a determination error occurs in image area separation processing, the technique can fail to apply an identical image processing scheme for an identical object, resulting in appearance of intermittent staircasing and unnatural object boundaries in an output image.